In a conventional single-sensor camera using an image sensing device such as a CCD, a color filter of one of a plurality of types (e.g., one of three colors R, G, and B) is formed for each pixel of the image sensing device. For example, the Bayer arrangement shown in FIG. 1 is well known.
In a single-sensor camera having this Bayer arrangement, color information of only one of R, G, and B is obtained by each pixel. To obtain color information of all of R, G, and B in each pixel, therefore, interpolation must be performed in a subsequent circuit. The simplest interpolation method is to set 0 as all color signal levels except for a color signal to be interpolated, and perform two-dimensional, low-pass filter (LPF) processing for each pixel, thereby obtaining an interpolated value.
Unfortunately, if this two-dimensional LPF processing alone is simply performed as described above, high-frequency components are lost from the frequency characteristics of the interpolated image, and this blurs the image.
To solve this problem, a method by which the LPF processing is changed in accordance with the characteristics of an image has been proposed. For example, if a certain portion of an image is found to be a vertical stripe, the LPF processing is performed only in the lateral direction of the image; if a certain portion of an image is found to be a lateral stripe, the LPF processing is performed only in the longitudinal direction of the image. If a certain portion of an image is found to be a wave intermediate between vertical and lateral stripes, the LPF processing in the lateral direction of the image and the LPF processing in the longitudinal direction of the image are weighted in accordance with the degree of the wave, thereby interpolating a color signal.
However, the above conventional method does not take into account the sensing of an image (e.g., a vertically striped pattern or laterally striped pattern) having a frequency close to the Nyquist frequency of the image sensing device.
A frequency close to the Nyquist frequency of the image sensing device is a folded signal, so it is difficult to determine whether the image is a vertical or lateral stripe. This may lead to erroneous determination between vertical and lateral stripes. For example, although an actual object is a lateral stripe, the longitudinal LPF processing may be performed by mistake for a certain pixel to be interpolated, whereas the lateral LPF processing is performed for the next pixel to be interpolated. That is, appropriate color signal interpolation may not be performed continuously.